How to modify the built-in equations in Comsol Multiphysics

Hello everyone,

I want to modify the built-in equations in Comsol Multiphysics, but always encounter problems. For example, I want to add a starting pressure gradient in Darcy's law in Porous Media Module, but the system prompts an error. The lamp is a variable (starting pressure gradient) defined by me, as shown in attached files. Thank you very much.